In various vehicles including automobiles, seat belt devices for protecting occupants in emergency such as collision are attached to seats thereof. In order to facilitate the occupant putting on and taking off the seat belt, a buckle device is normally provided. In general, this buckle device is configured so that a latch member for latching a tongue plate is biased by a spring in a latching direction for the tongue plate, and that the latch member is kept in a latching state between the tongue plate and a buckle main body by a lock member.
In such the buckle device, it is necessary to display the latching state between the tongue plate and the buckle main body by means of a lamp, to control the operation of a belt retracting force reducing mechanism such as a retractor, or to enable seat belt wearing on/off information to be transmitted to an air bag ECU (electronic control unit). Therefore, inside the buckle main body, there is provided a buckle fastening detector for detecting a latching state (refer to, for example, in Japanese Patent un-examined Publication JP-A-2003-81057—hereinafter “Japanese Publication 1”).
As the buckle fastening detectors, a non-contact type detector including a Hall effect semiconductor element and a magnet, a contact type detector including a substrate having print-wired electrodes and a slider including a contact member which slides on the substrate are known. In the contact type buckle fastening detector described in Japanese Publication 1, as shown in FIG. 9 thereof, a normally-closed contact NC, a common contact COM, and a normally-opened contact NO are arranged in order on a rectangular substrate (100) with print-wired respectively in the rectangular shape. A slider (101) which slides with insertion of a tongue plate includes therein a contact member (102) having a contact (102a) abutting on the normally-closed contact NC or the common contact COM, and a contact (102b) abutting on the common contact COM or the normally-opened contact NO. In the non-latching condition, as shown by solid lines in FIGS. 9 and 10, the slider (101) connects electrically the common contact COM and the normally-closed contact NC. In the latching condition, as shown by a dashed dotted line in FIG. 10, the slider (101) connects electrically the common contact COM and the normally-opened contact NO. On the upper surface of the slider (101), an engagement protrusion (103) is formed, which fits into an engagement hole provided in a leg portion of an ejector (not shown).
Further, in another contact type buckle fastening detector shown in FIG. 11, of the Japanese Publication 1 a normally-closed contact NC and a normally-opened contact NO are arranged on a rectangular substrate (100) in parallel with a common contact COM, and their contacts are print-wired respectively in the rectangular shape. A slider (101), similarly to the above slider, includes therein a contact member (102) having a contact (102a) abutting on the common contact COM and a contact (102b) abutting on the normally-closed contact NC or the normally-opened contact NO. In the non-latching time, as shown by solid lines in FIGS. 11 and 12, the slider (101) connects electrically the common contact COM and the normally-closed contact NC. In the latching condition, as shown by a dashed dotted line in FIG. 12, the slider (101) connects electrically the common contact COM and the normally-opened contact.
In the seat belt buckle fastening detector described in the Japanese Publication 1, when the electric connection with the contact member of the slider is switched from the normally-opened contact NO to the normally-closed contact NC, as shown by chain double-dashed lines in FIGS. 10 and 12, there is a gray zone in which the slider neither contacts electrically with r the normally-closed contact NC nor the normally-opened contact NO. In order to reduce the width of this gray zone, it is desirable to reduce a surface distance between the normally-opened contact NO and the normally-closed contact NC. However, if the surface distance between the normally-opened contact NO and the normally-closed contact NC is reduced, there is high possibility that short-circuit is produced between the normally-opened contact NO and the normally-closed contact NC due to attachment of waterdrops (e.g. condensations of water).